a. Field of the Invention
The invention relates generally to methods for making photomasks and more particularly to methods for making thermochromic photomasks.
b. Prior Art
The process of manufacturing integrated circuits involves several distinct steps: (i) circuit design, (ii) reticle plate generation, (iii) master photomask preparation, (iv) production of sets of photomasks from photoplates, (v) wafer fabrication, and (vi) wafer slicing, testing and packaging.
Circuit design defines the electrical characteristics of the integrated circuit to be contained on a chip of silicon, and determines the number of photomasks, usually about seven, necessary to produce a complete integrated circuit. Several hundred identical integrated circuits can be manufactured simultaneously on a thin disc of silicon, called a "wafer". A circuit design drawing is usually prepared by the integrated circuit manufacturer. The design drawing is then employed for reticle plate generation and photomask production.
This circuit design drawing is translated to digital information on magnetic tape which is then used to generate a set of individual high precision reticle plates for each integrated circuit. These reticle plates are used to produce master photomasks which contain the design details of electronic circuit elements which will be transferred to a silicon wafer.
The digital information on the magnetic tape controls a precision pattern generator which produces the reticle plates. The negative of the circuit image which appears on the reticle plate is further reduced to actual size and is photographically repeated a like number of times on a single photoplate to produce a master photomask. The master photomask is approximately the same size as the silicon wafer to be processed. Each master photomask is utilized through high-precision photographic printing techniques to produce many photomasks which are exact reproductions of the master photomask and are used in the wafer fabrication of integrated circuits.
Wafer fabrication by the integrated circuit manufacturer consists of a series of chemical and physical processes in which the photomask images are transferred to the wafer, which is typically three inches in diameter and coated with ultraviolet radiation sensitive photoresist. Exposure to high-intensity ultraviolet radiation results in the transfer of the images on the photomask to the wafer. This is accomplished in a room lighted by a yellow-orange light to avoid exposing the photoresist coating the wafers to any ultraviolet light. Additional chemical processing, including etching, and the selected introduction of chemical impurities impart to the wafer the properties necessary to form electronic elements such as transistors. This process is repeated with each of the individual photomasks in a defined sequence, to produce all of the required parts of the many identical integrated circuits being produced on the wafer. Wafer fabrication is completed by the precision etching of a layer of aluminum previously deposited on the wafer, which results in the desired pattern of electrical interconnections of the numerous electronic elements contained in each of the individual integrated circuits.
A completed wafer may contain 500 identical integrated circuits. Each individual integrated circuit is tested and those not meeting specifications are marked for rejection. The individual integrated circuits are separated from the wafer and the rejected integrated circuits are discarded. Each remaining integrated circuit, or chip, is individually assembled and mounted in a circuit interconnection package.
Previously, it was recognized that there was an advantage in using photomasks having image areas which were transmissive in yellow-orange spectral regions while the same areas were opaque to ultraviolet light. The advantage of such a mask is that it may be readily aligned visually in a yellow-orange lighted room by an operator, with previously deposited patterns on a wafer.
A sequence of several masks is used in the manufacture of integrated circuits and sometimes masks must be superposed over wafer patterns which may only partially define larger patterns. It is typical that masks which are used to develop subsequent wafer patterns must be aligned with previously developed and processed wafer patterns made with other masks. One type of yellow-orange transparent mask of the prior art in which alignment is easier is the iron oxide mask. In this type of mask a glass substrate has a thin iron oxide coating which in turn is coated with a positive or negative photoresist layer which is exposed by light to an image pattern. The exposure changes the solubility of the photoresist so that a developer can remove unwanted photoresist portions. Underlying iron oxide portions may then be etched away through the opening in the photoresist.
The remaining mask photoresist may be stripped away, leaving islands of iron oxide forming the desired wafer masking patterns. The iron oxide image areas on glass are transmissive to yellow-orange light, but essentially opaque to the ultraviolet used, while the clear areas are transparent to visible and the ultraviolet light used. While this type of photomask has advantages, it is considered to be costly, presently selling for approximately twenty-five dollars per finished mask, compared to about four dollars for a silver-halide emulsion mask.
In U.S. Pat. No. 4,004,925 Van Besauw et al. teach the advantage of transparent emulsion masks. The patent describes a post processing chemical technique in which diazonium, pyrylium or thiapyrylium salts combine with silver-halides to form image areas transparent to visible light of wavelengths longer than 500 nanometers but opaque to ultraviolet in an emulsion mask.
N. Chand, in U.S. Pat. Nos. 3,567,447 and 3,639,125, teaches that while heating a processed silver-halide emulsion photomask to achieve differential solubility, the non-image areas (clear gelatin) darken to a reddish color. This darkening is thought to be a charring of the clear gelatin. In the reference Chand patents the clear gelatin is charred after being heated for one hour and apparently becomes more soluble in certain chemicals.
In U.S. Pat. No. 2,911,749 Stookey teaches the making of photographs on glass using a preferred temperature of 525.degree. C. for more than one hour. The image areas are opaque.
In U.S. Pat. No. 3,406,066 Avery teaches the conversion of photographic images to metallic ferrocyanides which yield colored oxides on firing at a temperature greater than 1,000.degree. C. for an apparently long duration. The image areas are opaque.
In U.S. Pat. No. 3,664,837 Stanley teaches production of line patterns on glass plates by heating to a temperature of between 400.degree. C. and 500.degree. C. for an unknown duration. The image areas are opaque shiny silver.
In U.S. Pat. No. 3,674,484 Spinski teaches production of photographic images on ceramic by heating to a temperature range between 670.degree. C. and 815.degree. C. for hours. The image areas are opaque black silver.
On another point in the prior art, the literature states that during the interaction of silver ions with gelatin in a homogeneous medium, there are formed thermally-stable complex compounds of silver ions and gelatin. It is unknown whether the silver is adsorbed into the gelatin or whether a compound of some type is formed. These materials are referred to as silver-gelatin complexes. (The Chemistry of Photographic Mechanisms by K. L. Lyalikov, Focal Press, 1967, pages 274-275.)